There is known a shift control apparatus for an automatic transmission as recited in Published Japanese Patent Application, Kokai No. 2001-132835, for example.
This publication discloses technique designed to achieve smooth shift and reduce a shift time in a so-called dual changeover shift in which two or more friction elements are released and two or more friction elements are engaged at a time, such as a shift from a sixth speed attained by engagement of a first friction element and a second friction element, to a third speed attained by engagement friction elements (third and fourth friction elements) other than the first and second friction elements.
Specifically, this technique is designed to start the release of the first friction element form the state of sixth speed, then to start the release of the second friction element, and to complete the engagement of the fourth friction element after the completion of engagement of the third friction element. By so doing, this technique achieves a smooth shift with a reduced shift time in a continuous manner having no two-step form without establishing an intermediate shift speed which can be established between the sixth speed and the third speed, by engagement of the second friction element and the third or fourth friction element.
However, the automatic transmission shift control apparatus of the earlier technology performs a so-called reshift inhibition control for inhibiting a change of a target shift speed from an end of a predetermined time after a start of the shift until the end of the shift control with the attainment of the final shift speed, in order to prevent shift shock which would be caused normally by change of friction elements under the control and abrupt change of oil pressure in a series of shift control operations. Therefore, if the driver varies the accelerator depression quantity, and hence the shift speed on a shift diagram differs from the final shift speed of the dual changeover shift in progress, it takes a considerable amount of time to finally attain the shift speed on the shift line conforming to the driver's intention.
This problem is explained by the use of a time chart of FIG. 11. After the 6-3 shift is started at instant to, and the change of the target speed of 6-3 shift is inhibited at instant t2, a 6-4 shift request is produced at instant t3 in response to crossing across a 3-4 upshift line due to a change of the accelerator depression quantity by the driver. Since the change of the target shift speed inhibited, the 6-3 shift is continued. At te4, the third speed is established, the change of the target speed is permitted, and the fourth speed request is accepted. Then, the 3-4 shift is performed and completed at instant t5. Thus, in spite of the driver's request to change the target speed at instant t2, the shift according to the driver's intention is achieved at instant t5. It takes a very long time to attain the speed according to the driver's intention.
In view of the above-mentioned problem, it is an object of the present invention to provide an automatic transmission shift control apparatus capable of responding to a driver's request quickly and achieve a dual changeover shift smoothly, even when the driver changes the shift intention and requests a shift to another speed during the process of shift achieved by a plurality of changeover control operations, after the change of target speed is inhibited.